Fix flash when canceling the recents animation with a screenshot

- When canceling because of starting home, use the screenshot call to
  allow Launcher to handle the cancel more cleanly
- Since the cancel+screenshot path was added, we no longer cancel
  on root task order change (we just get onTaskAppeared now), it's
  purely a deferred path for the runner to handle the cancel in a clean
  way. So instead of having a separate animator, just add a screenshot
  surface to the existing leash, and clean up everything when the
  runner finishes the recents animation. This prevents the issue where
  transferring the animation causes the task to be reparented from the
  leash and made immediately visible (and fullscreen)
- Remove some unused code

Fixes: 185643608
Test: atest RecentsAnimationControllerTests
Test: Manually enable live-tile for 2/3 button, tap home and ensure
      there's no flash
Change-Id: I57775e5df55ae80931ad02c21f190f29ffa76881